Portion of a top surface of an exercise device



FIG. 1 is a top perspective view of a portion of a top surface of an exercise device showing my new design; and,

FIG. 2 is a side view thereof, the opposite side being a mirror image thereof.

The even broken lines showing portions of a top surface of an exercise device are environmental only and form no part of the claimed design. 

The ornamental design for a portion of a top surface of an exercise device, as shown and described. 